Field of the Invention
The present invention relates to synthetic biology, especially using operons and synthetic constructs to produce a non-natural or engineered cyclic by-pass of the photorespiration pathway in organisms such as plants, cyanobacteria and microbes.
Related Art
Oxygenic photosynthesis is the primary source of nearly all biological energy. In this process, light is converted into chemical energy which is used to fix CO2 in the CB cycle through the enzyme RuBisCO. The carboxylase activity of RuBisCO results in the addition of one molecule of CO2 to one molecule of ribulose-1,5-bisphosphate to create two molecules of 3-phosphoglycerate, thus fixing inorganic CO2 into triose phosphates. However, the competing oxygenase activity of RuBisCO results in the loss of fixed carbon through a process termed photorespiration. One of the ‘holy grails’ of photosynthesis research has been to engineer RuBisCO to improve CO2 fixation and reduce photorespiration; however, these attempts have been met with limited success. It has been shown that biochemical constraints as well as abiotic factors are crucial considerations in addressing the protein engineering of RuBisCO (1,2). Given this complexity, a more promising approach may be to accept the inherent ‘flaws’ of RuBisCO and improve net photosynthetic rates through engineered photorespiratory bypasses.
The role of photorespiration is highly debated, as it consumes much more energy and cellular resources than its carboxylase counterpart reaction. The fixed O2 from RuBisCO results in the toxic intermediate, 2-phosphoglycolate, which continues through the photorespiratory pathway (C2 cycle). This pathway is costly, because the 2-phosophoglycolate must be metabolized in order to detoxify the cell through an elaborate pathway involving more than a dozen enzymes (CHECK). Furthermore, the glycine decarboxylase conversion of glycine to serine, in the C2 cycle, releases both an ammonia and a CO2 molecule, resulting in a net loss of carbon and nitrogen. Previous work has bypassed the C2 cycle by introducing the glycolate catabolic pathway from Escherichia coli into Arabidopsis thaliana chloroplasts resulting in improved growth rates (2). The pathway introduced by Kebeish et al circumvented the loss of nitrogen; however, the glyoxylate carboligase decarboxylates glyoxylate, losing one CO2 molecule and thus still resulting in a net loss in carbon. Although the irrelevance of photorespiration can be inferred from this work, genome-scale metabolic modeling of cyanobacteria has suggested that photorespiration is essential for optimal photosynthesis (3).
Photorespiration produces the toxic intermediate 2-phosphoglycolate, which is recycled through the photorespiratory C2 cycle (FIG. 1A). This pathway is costly, requiring ATP and reducing equivalents in an elaborate reaction sequence involving more than a dozen enzymes and transporters. Furthermore, the reaction catalyzed by glycine decarboxylase, converting two glycine molecules into one serine in the C2 cycle, releases both NH3 and CO2, resulting in a net loss of carbon and nitrogen. To date, only two studies have attempted to experimentally decrease the negative impacts of the photorespiratory C2 cycle by expression of alternative glycolate metabolic pathways. Kebeish et al. (3) attempted to bypass most of the C2 cycle by introducing the glycolate catabolic pathway from E. coli (4) into Arabidopsis thaliana chloroplasts. This pathway circumvents the loss of nitrogen, but the glyoxylate carboligase reaction results in the release of one CO2 per two glyoxylate molecules (FIG. 1A). Although increased biomass was reported, interestingly, transformants expressing only the first enzyme of that pathway, glycolate dehydrogenase, showed similar results, rendering the approach controversial. In a second study, Maier et al. (5) introduced a glycolate oxidation cycle into Arabidopsis chloroplasts; however this pathway results in the release of even more CO2 than the heterologously expressed glycolate catabolism pathway. In both cases, CO2 release occurs in the chloroplast, where it can potentially be refixed by RuBisCO. The challenges associated with designing experimental approaches to mitigate the losses associated with photorespiration are likewise underscored by results from systems-level genome-scale metabolic modeling that suggests photorespiration is essential for optimal photosynthesis (6)
Introduction of additional, synthetic CO2 fixation pathways provide an approach to increasing photosynthesis, which circumvents the complexities associated with manipulating the C2 cycle (7). Of the six known CO2 fixation cycles in nature, only the 3-hydroxypropionate (3OHP) bi-cycle is completely oxygen insensitive (8,9), a key consideration when engineering pathways into oxygenic photoautotrophs. The 3OHP bi-cycle from the thermophilic anoxygenic phototroph Chloroflexus aurantiacus offers an attractive starting point for engineering efforts (10), because all of the necessary enzymes have been characterized (9). In this bi-cyclic pathway, bicarbonate is fixed by biotin-dependent acetyl-CoA carboxylase and propionyl-CoA carboxylase. The primary CO2 fixation product resulting from the first cycle is glyoxylate, which is then fed into the second cycle, in which another bicarbonate is fixed and pyruvate is generated as the final product (9).
Independent of photorespiration, various synthetic carbon fixation pathways have been proposed as a potential way to increase net photosynthetic yield (4). Of the six known carbon fixation cycles that exist in nature, only the Calvin-Benson cycle and the 3-hydroxypropionate bicycle lack enzymes that are oxygen sensitive (5), a key factor to consider when engineering pathways into oxygenic photoautotrophs. Further studies have expanded upon natural carbon fixation pathways to predict novel carbon fixation pathways by mining enzyme databases and building cycles in silico (6).
Carbon and carbon dioxide (CO2) fixation in cyanobacteria proceeds via the reductive pentosephospate cycle (Calvin-Benson cycle). The key carboxylase of that CO2 fixation cycle is ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The oxygenase side reaction of RuBisCO results in the formation of a toxic compound (2-phosphoglycolate), which has to be removed or ideally recycled. In cyanobacteria and plants this is achieved in a series of reactions (photorespiration) involving the loss of CO2 and NH3, which both have to be re-assimilated at the cost of additional energy.
Cyanobacteria convert CO2 into biomass using solar energy. The rate limiting step in this process is the fixation of CO2 by enzyme RuBisCO. RuBisCO is a very inefficient catalyst, because it has relatively low affinity to its substrates and is not able to discriminate between CO2 and O2. The use of O2 instead of CO2 leads to photorespiration. Phosphoglycolate produced from the oxygenase side reaction of RuBisCO is toxic to cells, because it completely inhibits triosephosphate isomerase at micro molar levels. Therefore, 2-phosphoglycolate is recycled to 3-phosphoglycerate via a series of reactions (FIG. 1). This leads to the release of CO2 and NH3, which both have to be re-fixed consuming extra energy.
Others have described different methods of increasing photosynthetic carbon fixation. For example, Kebeish, Rashad et al. describe a method for increasing photosynthetic carbon fixation in rice in WO2010012796 A1, hereby incorporated by reference. Kreuzaler, Fritz et al. describe a method for increasing photosynthetic carbon fixation using glycolate dehydrogenase multi-subunit fusion protein in WO2011095528 A1, also hereby incorporated by reference. However, there is a need for other compositions and methods that provide more energetically and metabolically desirable approaches.